There are many substances which have very small vapor pressures, but whose presence in air is nonetheless undesirable because they are very toxic or indicate the presence of unwanted substances such as explosives, drugs, etc. Consider the explosives 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (2,4-DNT), and 2,6-dinitrotoluene (2,6-DNT). The saturation concentrations of these substances in air at room temperature are approximately 10 ppb, 300 ppb, and 700 ppb, respectively, numbers large enough to suggest that 2,4- and 2,6-DNT can be detected using existing techniques. However, in the real world, a terrorist is unlikely to present the detector with a volume of saturated air. At best, the fraction of molecules available to a xe2x80x98snifferxe2x80x99 will be further reduced by a few orders of magnitude. Therefore, detectors must be able to detect these explosives at vapor concentrations a few orders of magnitude less than their saturation concentrations.
We here disclose the development of a novel detection method using photoinduced nucleation and the demonstration of its ability to detect and quantify TNT, 2,4-DNT, and 2,6-DNT in humid air at concentrations as small as 5 parts per trillion (ppt), 10 ppt, and 10 ppt, respectively. Thus Photo Induced Nucleation Detection (PIND) can be used to detect these explosives, even if their concentration at the detector is 2000, 30000, and 70000 fold smaller than their saturation concentrations.
The phenomenon that upon irradiation (illumination) with light of suitable wavelength and intensity, certain organic compounds (e.g., o-tolualdehyde) (OTA) cause very efficient nucleation of supersaturated vapors has been previously reported. This phenomenon (which was named photoinduced nucleation) could be utilized to detect and identify substances, even when they are present in very low concentrations. However, the experimental device then used was not designed to sample substances in air. The substances to be detected were placed in a liquid pool from which molecules evaporated to give a known vapor concentration. The substances could not be introduced from outside of the device since molecules from a vapor injection would diffuse to the liquid pool, resulting in sample losses and contamination of the pool. Also, in the above-mentioned work, the photoinduced nucleation occurred by illuminating (with UV light) the substance being detected when it was in a supersaturated vapor. Therefore, it was unknown whether the presence of a vapor which was supersaturated played an essential role in the photoinduced nucleation process.
It is an object of the invention to provide a method and apparatus to detect molecules of substances in the air at extremely low concentrations of less than about 100 ppb.
Another object of the invention is to provide a method and apparatus to detect molecules of substances such as pollutants and explosives in concentrations of a few ppt.
Still another object of the invention is to provide a method and portable apparatus for using supersaturated vapors to increase the size of photoproducts produced by exposing air containing detectable substances to light to produce photo excited molecules.
A still further object of the invention is to identify the presence, density and concentration of detectable substances at very low concentrations.
The invention consists of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.